1. Field of the Invention
The present invention relates to a linearity-improved Doherty amplifier apparatus.
2. Description of the Related Art
In conventional common amplifiers for amplifying the power of a radio frequency signal, such as a code division multiple access (CDMA) signal or multi-carrier signal, the operation range is widened to the region near the saturation region, using a distortion compensator, thereby reducing the power consumption. There are limits to reducing the power consumption of the common amplifier only using a feedforward distortion compensator or pre-distortion compensator. In recent years, attention has been paid to Doherty amplifiers as highly efficient amplifiers.
In a conventional Doherty amplifier as shown in FIG. 1, an input signal from an input terminal 1 is divided into two signals by a divider 2. One of the resultant signals is input to a carrier amplifier 4. The signal output from the carrier amplifier 4 is guided to a node 62 via a λ/4 transformer 61 for impedance transformation.
The other signal output from the divider 2 is supplied to a phase shifter 3, where the phase of the signal is shifted by 90°. The resultant signal is input to a peak amplifier 5, which, in turn, outputs a signal to the node 62.
The output signals of the carrier amplifier 4 and peak amplifier 5 are combined by a Doherty synthesis unit 6 formed of the λ/4 transformer 61 and node 62. The resultant signal is output to an output terminal 8 via a λ/4 transformer 7 for impedance transformation. The output terminal is connected to a load 9.
An amplifier element 42 incorporated in the carrier amplifier 4 is biased as class AB, while an amplifier element 52 incorporated in the peak amplifier 5 is biased as class B or C. The amplifier element 42 operates singly until the input signal reaches a certain level at which the peak amplifier 5 operates. When the amplifier element 42 enters its saturation region and its linearity starts to deteriorate, the amplifier element 52 starts to operate. At this time, the output signal of the peak amplifier 5 is input to the load 9, whereby the peak amplifier 5 drives the load 9 along with the carrier amplifier 4. Further, at this time, the load line of an output matching circuit 43 shifts from a higher impedance to a lower impedance. However, since the amplifier element 42 is in its saturation region, the Doherty amplifier exhibits high efficiency. When the level of the input signal is further increased, the amplifier element 52 also enters its saturation region. At this time, since both the amplifier elements 42 and 52 are saturated, the Doherty amplifier exhibits high efficiency.
FIG. 2 illustrates the theoretical collector or drain efficiency of the Doherty amplifier shown in FIG. 1, and the efficiency of a general class B amplifier. The collector efficiency is defined as the ratio of the radio frequency output power output from the collector of an amplifier transistor to the product of a DC voltage applied by a power supply to the collector and a DC current supplied from the power supply. The drain efficiency is also defined in that way. The horizontal axis of FIG. 2 indicates the amplifier back-off, i.e., the dB ratio between a compression point of 0 dB and the input level of the amplifier. The compression point is defined as the minimum input signal level at which the amplifier elements 42 and 52 are saturated.
When the input signal level is in range A, only the carrier amplifier 4 operates, in general. When the amplifier back-off reaches about 6 dB, the carrier amplifier 4 starts to be saturated, and the efficiency of the Doherty amplifier reaches about the maximum efficiency of the class B amplifier. At this time, the output power of the carrier amplifier 4 is about P0/4, assuming that the maximum output power of the Doherty amplifier is P0.
When the input signal level is in range B, the output of the carrier amplifier 4 increases from about P0/4 to about P0/2 and that of the peak amplifier 5 increases from about 0 to P0/2, as the input signal level increases. At this time, the sum of the outputs of the carrier and peak amplifiers 4 and 5 increases in accordance with an increase in the power input to the input terminal 1, with the same proportionality constant as in range A. Although the efficiency once decreases when the peak amplifier 5 starts to operate, it again assumes the peak value at the compression point at which the peak amplifier 5 also starts to be saturated. At the compression point, the output of the carrier amplifier 4 is substantially equal to that of the peak amplifier 5.
In general, the CDMA signal or multi-carrier signal has a high peak factor (the ratio of the peak power to the average power) is high. Accordingly, in general amplifiers, to be compatible a peak factor of 7 to 12 dB, the point acquired by subtracting the peak factor of 7 to 12 dB from the compression point is used as their operating point.
A description will now be given of impedance transformation performed in the λ/4 transformers 7 and 61. Since the impedance of output load Z0 is set constant, it is used as a start point. Assuming that the characteristic impedance of the λ/4 transformer 7 is Z2, impedance Z7 of the λ/4 transformer 7 seen from the node 62 is given byZ7=Z22/Z0 
Impedance Z4 of the λ/4 transformer 61 seen from the output matching circuit 43 is given in the same manner as impedance Z7, since the output impedance of an output matching circuit 53 is substantially infinite in range A. In range C, since the output matching circuits 43 and 53 bear the same load, the load impedance of the λ/4 transformer 61 (i.e., the contributory share of the carrier amplifier 4 at the node 62) and the load impedance of the output matching circuit 53 are both 2Z7, and the following equations are given:
      Z    4    =      {                                                                                                            Z                    1                    2                                                        Z                    7                                                  =                                                                            Z                      1                      2                                                              (                                                                        Z                          2                          2                                                /                                                  Z                          0                                                                    )                                                        =                                                            Z                      0                                        ⁢                                                                  Z                        1                        2                                                                    Z                        2                        2                                                                                                                                                (                                  range                  ⁢                                                                          ⁢                  A                                )                                                                                                                              Z                    1                    2                                                        2                    ⁢                                          Z                      7                                                                      =                                                      (                                          1                      /                      2                                        )                                    ⁢                                      Z                    0                                    ⁢                                                            Z                      1                      2                                                              Z                      2                      2                                                                                                                          (                                  range                  ⁢                                                                          ⁢                  C                                )                                                    ⁢                                  ⁢                  Z          5                    =              {                                            ∞                                                      (                                  range                  ⁢                                                                          ⁢                  A                                )                                                                                        2                ⁢                                  Z                  7                                                                                    (                                  range                  ⁢                                                                          ⁢                  C                                )                                                        where impedance Z1 is the characteristic impedance of the λ/4 transformer 61. In range B, impedances Z4 and Z5 shift between the values in range A and the values in range C.
If the conventional Doherty amplifier using the semiconductor amplifier element 42 is used for a high-frequency band, it is difficult to make the impedance seen from the amplifier element 42 coincide with that according to the Doherty theory. This is because the load line seen from the amplifier element 42 is varied by the behavior of the output matching circuit 43.
On the other hand, PCT National Publication No. P2005-516524A, HYEONR TAE JEONG; TAE HO KIM; IK SOO CHANG; CHUL DONG KIM; “A doherty amplifier with a bias adaptation technique based on SDR transmitter architecture”, Microwave journal, Vol. 48, No. 9, 2005, pp. 140-158, JP-A 2004-173231 (KOKAI) and JP-A 2004-96729 (KOKAI) disclose improved Doherty amplifiers, in which the bias applied to the peak amplifier is controlled to keep the amplifier element off in the low-power mode, and to operate it as class AB in the high-power mode.
However, it is assumed that these improved Doherty amplifiers exhibit the AM-AM characteristic shown in FIG. 3. In FIG. 3, solid line a indicates the characteristic of a Doherty amplifier having a bias control function, and broken line b indicates the characteristic of a general Doherty amplifier having no bias control function. In the case of the Doherty amplifier with the bias control function, in the region in which the input signal level is low and the peak amplifier does not operate, a loss of 3 dB occurs because of the divider. In contrast, in the region in which the input signal level is high and the peak amplifier operates as class AB, the output power of the standard class AB amplifier is synthesized without any distribution loss, whereby the gain of the Doherty amplifier is increased by 3 dB. Accordingly, it is considered that the conventional Doherty amplifier with bias control exhibits a degraded AM-AM characteristic. Actually, however, a gain increase of as much as 3 dB cannot be expected since the load of the carrier amplifier varies. For facilitating the description, the gain increase is tentatively set to 3 dB.
As described above, where the conventional Doherty amplifier shown in FIG. 1 is used for a high-frequency band, utilizing a semiconductor amplifier element, it is difficult to make the impedance seen from the amplifier element coincide with that according to the Doherty theory. Furthermore, if the conventional Doherty amplifier is improved into such a highly efficient amplifier as disclosed in JP-A 2004-173231 (KOKAI) and JP-A 2004-96729, an extremely highly efficient amplifier can be acquired. In this case, however, the distortion characteristic is inevitably degraded.